MATERIAL HANDLING—TRANSPORTATION OF SOLIDS 97 Screws for ordinary service customarily consist of steel ribbons wound helically around the central steel shaft or pipe, but for severe service and handling heavy loads the flights are made much heavier, usually of steel, but occasionally of cast iron. The screws are supported by a series of bearings spaced from 6 to 10 ft. centers and the troughs are closed at the charging end. The standard pitch is approximately equal to the diameter of the screw. Load is discharged through the open far end of the conveyor or along the conveyor through bottom gates in the trough. The drive is customarily located at the charging end of the conveyor, but when this is not feasible it may be located at the discharge end. Speeds of screw conveyors are not governed so much by the character of the load they handle as by their diameter and it has become customary to consider speeds approximating those given in Table 10 as standard for almost all materials handled, by screw conveyors. TABLE 10.—ECONOMIC SPEEDS FOB SCREW CONVEYORS Diameter screw, inches Revolutions per minute Diameter screw, inches Revolutions per minute 3 240 10 160 4 220 12 150 5 205 14 140 6 195 16 130 7 185 18 120 8 175 20 115 9 170 24 100 Capacity. — A general formula for ascertaining the capacity of standard screw conveyors handling various materials of known weight is given as Formula 15. x_r, Where, F°rmula (15) W = Capacity in tons per hour, d = Diameter of screw in inches, R = Revolutions of screw per minute, and w' = Weight of material handled in pounds per cubic foot. The consumption of power by screw conveyors is relatively high, but they are usually short—a series of conveyors discharging from one to the other being used if the distance the material has to be handled is more appreciable—and have quite limited capacity, so the power consideration is not of great importance. The additional power required to lift the load should the conveyor be installed at an inclination is consequently not so marked. The inclination is limited to 10 or 15 deg. with the horizontal. A convenient formula for ascertaining power requirements of screw conveyors follows. Horsepower = 0.68TFL + WH 1,000 Formula (16) Where, W - Load handled in tons per hour, L = Length of conveyor in feet, and H s= Elevation in length of conveyor in feet.